callStack and callback

• what is call stack?
  • The mechanism the JS interpreter uses to keep track of its place in a script that calls multiple functions.
  • How js knows what functin is currently being run and what functions are called from within that function.
• How does call stack work?
  • When a script calls a function, the interpreter adds it to the call stack and then starts carrying out the function
  • any functions that are called by that function are added to the call stack further up, and run where their calls are reached
  • when the current function is finished, the interpreter takes it off the stack and resumes execution where it left off the last code listing.
  • key ided: last in, first out

  • const multiply = (x, y) => x * y;
    
    const square = x => multiply(x, x);
    
    const isRightTriangle = (a, b, c) => (
        square(a) + square(b) === square(c)
    )
    console.log("BEFORE")
    isRightTriangle(3, 4, 5)
    
    console.log("DONEEEE!")

    square(a) -> multiply(a,a) ，要想执行square，得先执行multiply，multiply执行完毕之后从栈顶pop出去，然后再执行square(a)。

• WHILE JS is SINGLE THREADED.单线程
  • At any given point in time, that single JS thread is running at most one line of JS code.
• What happens when something takes time?
  • we have workaround

  • console.log("Sending request to server!")
    setTimeout(() => {
        console.log("Here is your data from the server...")
    }, 3000)
    console.log("I AM AT THE END OF THE FILE!")
    
    >Sending request to server!
    >I AM AT THE END OF THE FILE!
    >Here is your data from the server...

  • Browsera come with WEB APIs that are able to handle certain tasks in the background(like make=ing requests or setTimeout)
  • The JS call stack recognizes these Web API functions and passes them off to the browser to take care of
  • Once the browser finishes those tasks, they return and are pushed onto the stack as a callback
• call back hell: 不停的嵌套setTimeOut

• ===============
  YIKES!!!!!!!!!!!
  ===============
  setTimeout(() => {
      document.body.style.backgroundColor = 'red';
      setTimeout(() => {
          document.body.style.backgroundColor = 'orange';
          setTimeout(() => {
              document.body.style.backgroundColor = 'yellow';
              setTimeout(() => {
                  document.body.style.backgroundColor = 'green';
                  setTimeout(() => {
                      document.body.style.backgroundColor = 'blue';
                  }, 1000)
              }, 1000)
          }, 1000)
      }, 1000)
  }, 1000)

  代码又丑又麻烦！！！于是，就有了promise这个东西